The present invention is directed to a process for the recovery of uranium from phosphoric acid. A solvent extraction process is employed to remove ionic metallic impurities, such as Ca.sup.+2, Mg.sup.+2, Al.sup.+3, and Fe.sup.+2,+3, and organic impurities from unpurified phosphoric acid containing uranium, preferably unpurified wet process phosphoric acid prepared by the dihydrate, the hemihydrate or the anhydrite process. The thus pretreated phosphoric acid contains uranium values. Uranium is recovered from pretreated phosphoric acid by means of another solvent extraction process. The uranium-depleted phosphoric acid can then be employed in the production of high quality superphosphoric acid (SPA).
Phosphoric acid containing uranium values is produced in the treatment of phosphate rock found in Florida and other parts of the United States and in several foreign countries. Phosphate rock is mined principally for conversion to superphosphoric acid (SPA) and thereafter to high phosphate fertilizer products. Mined phosphate rock is digested with sulfuric acid to produce a phosphoric acid solution (wet process phosphoric acid) and insoluble calcium sulfate (gypsum). The phosphoric acid solution is further processed to produce fertilizer products.
Phosphate rock mined in Florida contains uranium in low concentrations, about 0.01 to 0.02 weight percent in rock from central Florida, and somewhat less in rock from northern Florida. Although the concentration is low, great quantities of phosphate rock are converted to phosphoric acid each year. Wet process phosphoric acid having a P.sub.2 O.sub.5 content of 28 weight percent can have as much as 60 to 150 or more milligrams of uranium per liter. Such acid is a significant potential source of uranium. An economical process for recovery of uranium from such acid before it is converted to fertilizer and dispersed across the earth would be highly desirable.
A number of solvent extraction processes for recovery of uranium from wet process phosphoric acid have been described.
U.S. Pat. No. 2,835,552, the entire disclosure of which is incorporated herein by this reference, describes the extraction of uranium in reduced form with a phosphoric acid ester organic solvent, such as mono and diesters of orthophosphoric and pyrophosphoric acids and octyl alcohol dissolved in kerosene. Uranium is recovered as a solid tetrafluoride precipitate by extraction from the organic solvent with a solution of phosphoric acid and hydrofluoric acid.
U.S. Pat. No. 3,711,591, the entire disclosure of which is incorporated herein by this reference, describes solvent extraction of uranium in oxidized form with di(2-ethylhexyl) phosphoric acid (D2EHPA) and trioctylphosphine oxide (TOPO) dissolved in an organic diluent followed by reductive stripping of the uranium. The uranium is reoxidized in the stripping solution and then extracted therefrom with a second extraction solvent comprising D2EHPA and TOPO. Uranium is recovered from second extraction solvent as ammonium uranyl tricarbonate (AUT) by stripping with ammonium carbonate solution. The AUT is then calcined to U.sub.3 O.sub.8. The reoxidation and second extraction help to overcome problems of phase separation and hindered precipitation of uranium associated with humic acids coextracted from the phosphoric acid by the organic extractant.
U.S. Pat. No. 3,737,513, the entire disclosure of which is incorporated herein by this reference, describes solvent extraction of uranium from phosphoric acid with a dialkylphosphoric acid and a trialkylphosphine oxide dissolved in a water immiscible organic solvent. Uranium is reductively stripped from the organic extractant with an aqueous phosphoric acid or hydrofluoric acid solution containing ferrous ion.
U.S. Pat. No. 3,835,214, the entire disclosure of which is incorporated herein by this reference, describes the recovery of uranium from wet process phosphoric acid. Uranium in the phosphoric acid is reduced to the uranous form by contacting the acid with elemental iron. Reduced uranium is extracted with an organic extractant solution containing mono- and di-alkylphenyl esters of orthophosphoric acid, including octylphenyl esters, dissolved in an inert diluent such as kerosene. Uranium is oxidatively stripped from the organic extractant with an aqueous stripping solution containing phosphoric acid and an oxidant such as sodium chlorate. Oxidized uranium is extracted from the stripping solution with a second organic extractant comprising di(2-ethylhexyl) phosphoric acid and trioctylphosphine oxide dissolved in an inert organic diluent. The second organic extractant is scrubbed with water to remove phosphoric acid and is then stripped with an ammonium carbonate solution to form a solution or slurry of ammonium uranyl tricarbonate. The ammonium uranyl tricarbonate is calcined to U.sub.3 O.sub.8.
As is shown in the art, superphosphoric acid (SPA) is produced by conventional techniques of evaporation and/or dehydration of phosphoric acid. The P.sub.2 O.sub.5 content of the SPA so produced is from a minimum of about 65% P.sub.2 O.sub.5 to values over 100% P.sub.2 O.sub.5.
If unpurified phosphoric acid is used in the SPA process, SPA possessing a high viscosity is usually produced. On aging, a plastic-or-glass-like material results which is extremely difficult to handle. On the other hand, SPA produced from purified phosphoric acid does not exhibit these deleterious properties. Therefore, a number of processes have been described for the purification of phosphoric acid before conversion to superphosphoric acid.
One approach has been to selectively extract phosphoric acid from impure phosphoric acid, leaving behind a phosphoric acid solution having a higher concentration of the impurities. U.S. Pat. Nos. 3,367,749, 3,903,247, and 3,970,741 which are incorporated herein by reference, are exemplary. A disadvantage of this approach is that an even more impure phosphoric acid solution which contains uranium values is produced as a byproduct, and uranium must then be recovered from the very impure byproduct acid. Such impure byproduct acid can be treated for uranium recovery in accordance with practice of this invention.
Another approach for removing ionic metallic impurities has been to seed a concentrated wet process phosphoric acid solution to induce precipitation of ionic metallic impurities as insoluble salts and then to remove the precipitates by secondary filtration. Such processes are described in U.S. patent application Ser. No. 810,484, filed June 27, 1977, by Harold E. Mills, and titled "Metallic Ion Removal From Phosphoric Acid", and assigned to Occidental Chemical Co. now U.S. Pat. No. 4,136,199 The entire disclosure of said Application Ser. No. 810,484 is incorporated herein by this reference. Phosphoric acids treated by secondary filtration can be extracted to recover uranium by the process of the present invention.
It has also been recognized that it can be beneficial to purify wet process phosphoric acid before the acid is used as feed to a uranium solvent extraction process. In Recovery of Uranium From Wet-Process Phosphoric Acid by Extraction with Octylphenylphosphoric Acid by F. J. Hurst and D. J. Crouse, Ind. Eng. Chem. Process Des Develop., Vol 13, No. 3, pp 286-291 (1974), it is disclosed that organic matter present in phosphate rock and wet process phosphoric acid made therefrom can interfere with uranium recovery, causing variable results. Solids can build up at the aqueous-organic inerface in separators and cause flooding and unacceptably high solvent losses. Calcining of the phosphate rock before digestion in sulfuric acid is mentioned as a solution to the problem. Another solution mentioned is to age the acid to allow the organic matter to separate upon standing. However, such expedients do nothing to decrease the concentration of ionic metallic impurities in wet process phosphoric acid.
In the recent past, solvent extraction techniques for the removal of ionic metallic impurities from wet process phosphoric acid (in contrast with earlier processes for the extraction of pure phosphoric acid from impure acid) have been described. The extraction of ionic metallic impurities is disclosed to be for the purpose of purifying the wet process phosphoric acid before conversion of the acid to superphosphoric acid. The solvent extraction patents cited below do not describe the solvent extraction of uranium treated for removal of ionic metallic impurities.
Such a solvent extraction process has been described by Wiliams et al in U.S. Pat. No. 3,694,153 (hereinafter referred to as Williams) for the purification of wet process phosphoric acid. Using liquid water immiscible organic sulfonic acids dissolved in organic solvents, Williams was able to remove ionic metallic impurities from the phosphoric acid. Williams also described further treatment of phosphoric acid following the sulfonic acid treatment by contact with a water immiscible primary amine-solvent system for removing organic colorants and iron, aluminum, silicon, fluoride, and sulfate ions. The entire disclosure of U.S. Pat. No. 3,694,153 is incorporated herein by this reference.
Copending U.S. patent application Ser. No. 688,265, filed May 20, 1976, by Fernando Ore' and James L. Bradford, and assigned to the assignee of the present application, (now U.S. Pat. No. 4,053,564) is incorporated herein in its entirety by this reference. Said application Ser. No. 688,265 describes a solvent extraction process for the purification of phosphoric acid, particularly wet process phosphoric acid, using combinations of water immiscible organic sulfonic acids and water immiscible organic acid phosphates dissolved in a water immiscible organic solvent.
An embodiment of the process described in said U.S. patent application Ser. No. 688,265, now U.S. Pat. No. 4,053,564, employing dinonylnaphthalene sulfonic acid and di(2-ethylhexyl) phosphoric acid, was first practiced in a commercial scale plant on May 30, 1975. In Chemical & Engineering News, Sept. 1, 1975, page 22, it was reported that Fernando Ore' and James L. Bradford described a solvent extraction process for purifying phosphoric acid using "an aliphatic-substituted aromatic sulfonate as the active exchange agent and an alkyl phosphate ester as phase modifier, dissolved in a kerosene-type diluent", and that "the first commercial application of the . . . process is now in limited commercial operation at a phosphoric acid plant in northern Florida."
The processes of U.S. Pat. No. 3,694,153, and said U.S. patent application Ser. No. 688,265 are directed to the removal of ionic metallic impurities, notably Ca.sup.+2, Mg.sup.+2, Al.sup.+3, and Fe.sup.+2,+3 from wet process phosphoric acid. Phosphoric acid purified in accordance with these processes is disclosed to be useful in the manufacture of superphosphoric acid.